1. Field of the Invention
This invention relates to a system for minimizing the operating power required by a molding machine, such as an injection molding machine or a die casting machine, which has a constant speed electric motor supplied by alternating current power for driving hydraulic pumps for control of various operation functions of the machine. In particular, the system minimizes power applied to the motor during each of the various operating steps of a cycle of operation, for example of an injection molding machine, where such operation steps include one or more functions of the machine such as mold open, mold closed, inject, screw rotate, boost, and minimum speed.
2. Description of the Prior Art
Injection molding machines and die casting machines are typically hydraulically driven. Such machines will be referred to hereafter collectively as "plastic forming machines". An alternating current motor is provided with the machine to drive hydraulic pumps in order to provide pressurized hydraulic fluid to hydraulic cylinders for driving pistons which serve to perform various functions of the machine such as opening as mold, closing the mold, injecting plastic in the mold, rotating a screw during the mold closed function, providing boost hydraulic fluid during the injection function, and so on.
Most machines in commercial operation are provided with constant speed motors to drive, fixed volume hydraulic pumps. Such motors and pumps are sized to operate at the maximum capacity of the machine, yet for many articles made in the machine, maximum power of the machine is not required. Where the fixed volume pump produces more pressurized hydraulic fluid than required, excess fluid is returned via relief valves to the fluid supply, but power during each operating step of the machine and total energy is wasted by the motor operating at constant speed. Consequently, countless plastic molding and casting machines are in commercial operation which use more electric power for their operation than is required.